format string argument for metadata command

[acrisci]

Playerctl metadata queries should accept a format string like this:

$ playerctl metadata --format='%xesam:artist - %xesam:title'

This prints Katy Perry - Rise.

This covers what is probably the most common use case for formatting the metadata command.

[acrisci]

Started on this work in 8788d99 which implements this feature like

playerctl metadata --format 'now playing: {{artist}} - {{title}}'